The present relates generally to a low cost switching mode power supply and, more particularly, to such a power supply characterized by an extreme input voltage range, immunity to high input voltage transients, and stable operation.
One particular application for a low voltage power supply is to supply stable DC operating voltages for control circuitry of battery-powered vehicles, such as industrial vehicles, mining vehicles and over-the-road vehicles. These vehicles include large-capacity rechargable batteries, such as lead-acid batteries, supplying energy to electric motors through suitable heavy duty control circuitry. Typical motor control circuitry employs triacs or SCR's in various chopper circuits for high efficiency. More sophisticated vehicle power control systems implement a regenerative breaking techinque in order to partially recharge the batteries while braking the vehicle.
Such vehicles are manufactured in a wide range of configurations, varying greatly in power capability and battery voltage. For example, for various typical applications, nominal battery voltage may range from 24 volts to 144 volts. In addition to the wide range of battery nominal voltages, the terminal voltage of a particular battery varies widely during normal operating conditions. For example, although the nominal voltage of a single lead-acid battery cell is 2.0 volts, cell voltage may decrease to 1.0 volt for up to two minutes under extremely heavy load conditions, and may increase to 2.63 volts for one to five seconds during regenerative braking conditions. In addition, short term (e.g., 80 microsecond) repetitive transient voltage spikes of up to 460 volts, caused by the SCR chopper-controlled drive motor system and battery lead inductance, are present at the battery terminals.
In addition to the high capacity supplies for the drive motors in such electric vehicles, fairly low voltage (for example, 12 to 15 volt) DC supplies are needed for low signal level control functions, such as to implement a foot pedal accelerator control. In order to provide a steady, low voltage DC supply for control purposes, typical prior art electric vehicles include a relatively small battery, entirely separate from the batteries for supplying the vehicle motors. While such a solution is entirely acceptable from the standpoint of performance, it increases and complicates the effort required in keeping such vehicles maintained in operating condition. Not only must the main drive batteries be maintained in a charged condition, but the condition of the control battery must also be monitored, and the control battery must periodically either be recharged or replaced.
In those instances where power supplies have been employed in electric vehicles, the power supplies were closely matched to the input voltage supplied by the vehicle drive battery. For transient protection, such supplies typically employed a "brute force" filter comprising a series inductance and parallel capacitance.
While it is to these particular problems which the present invention is addressed, it will be appreciated that the power supply of the present invention is applicable to a variety of similar applications.
More particularly, the present invention is directed to the problem of providing a stable, low-voltage DC power suppy operable from a wide range of input voltages and transient conditions such as are described immediately above in the context of electric vehicles. There are two distinct aspects, both of which are addressed by the present invention.
First, while a wide range of such vehicles are manufactured including a wide range of nominal battery voltages for supplying the drive motors, the control circuits are quite similar, particularly insofar as their voltage requirements are concerned. Rather than expressly tailor a power supply to a single nominal battery voltage, thus requiring a large inventory of various different power supplies to meet the normal requirements, it is highly desirable that a single power supply operate well over a wide input voltage range such that it may be adaptable for use in a variety of different electric vehicles having a variety of nominal battery voltages, without concern for the selection of a particular power supply. By way of example, in accordance with the present invention, only two power supply models are required to cover the entire nominal battery voltage range of from 24 to 144 volts.
Second, the power supply must be capable of operating in the presence of extreme voltage transients such as are associated with a chopper-controlled DC motor.
As a specific example, a low input voltage range power supply operates from input voltages within the range of 12 to 111 volts, and withstands input transients of 230 volts. A high input voltage range power supply operates with input voltages within a range of 42 to 190 volts, and withstands input transients of 460 volts.
One particular form of known power supply is known as a switching mode "flyback" power supply wherein input voltage is supplied through a switching element to the primary winding of a power transformer such that current and magnetic flux gradually increase, and then the switching element is quickly opened to allow the magnetic field to rapidly collapse and induce a higher voltage than originally applied in either the same winding or a secondary winding. Flyback switching power supplies oscillate during operation. They are characterized by relative simplicity and consequent low cost. Since the oscillating frequency can be relatively high, for example within the range of 20 kHz to 200 kHz, a physically small and light weight transformer may be employed.
By way of example, one form of such a flyback mode power supply is disclosed in the Slack U.S. Pat. No. 3,911,352, entitled "Blocking Oscillator Type Power Supply with Power Foldback Short Circuit Protection." As the disclosure of that patent points out, such a power supply inherently has a certain degree of regulation as input voltages change. With higher input voltages, the current and flux in the primary winding have a greater rate of increase when the switching element turns ON, and the switching element turns OFF much sooner. The overall result is that a fairly constant amount of energy is delivered to the load circuit even in the face of input voltage changes. In addition, the Slack patent discloses an arrangement for monitoring the output voltage of the power supply, and for disabling the operation of the oscillator whenever input voltage exceeds a predetermined amount, thereby providing a form of output voltage reguation. In essence, the circuit of the Slack patent operates in bursts to maintain output voltage within a desired range.